PARTIAL COVERAGE OF PHOSPHOLIPID MODEL MEMBRANES WITH ANNEXIN-V MAY COMPLETELY INHIBIT THEIR DEGRADATION BY PHOSPHOLIPASE A(2)

Phospholipase A(2) (PLA(2))-mediated hydrolysis of membrane phospholip ids was measured by ellipsometry, and the inhibition of this process b y annexin V was studied, Planar membranes, consisting of phosphatidylc holine, phosphatidylethanolamine, and phosphatidylserine (PC/PE/PS; 54 :33:13, on molar basis), were degraded by pancreatic PLA(2), and the r ate of hydrolysis was limited to about 0.7%/min, The influence of grad ed coverage of the membrane with annexin V was studied, The degree of PLA(2) inhibition was nonlinearly related to the amount of membrane-bo und annexin V, and binding of only 12% and 54% of full membrane covera ge resulted in, respectively, 50% and 93% inhibition, These findings i ndicate that the inhibition of PLA(2)-mediated hydrolysis by annexin V cannot be simply explained by shielding of phospholipid substrates fr om the enzyme, Moreover, the present results leave room for a role of endogenous annexin V in regulating phospholipid turnover in the plasma membrane of parenchymal cells such as cardiomyocytes.